1. Field of the Invention
The present invention relates to a DC-DC converter capable of varying output voltage.
2. Description of the Related Art
FIG. 15 is a circuit diagram showing a series regulator or a DC-DC converter of prior art disclosed in page 51 of the Practical Power Circuit Design Handbook (published by CQ Publishing Co., Ltd. in May of 1988). In FIG. 15, 1 denotes a dc power supply for supplying input voltage, and 2, a transistor which is connected to the positive terminal of the dc power supply 1 for converting voltage. 3 is a high-potential output terminal of converted voltage, which is connected to the emitter of the transistor 2. 4 denotes a low-potential output terminal, which is connected to the negative terminal of the dc power supply 1.
5 and 6 are resistors for detecting output voltage, which are connected in series between the output terminals 3 and 4. 7 denotes a command power supply containing command voltage. 8 is a differential amplifier which compares the divided value of output voltage developed at the junction of the registers 5 and 6 with the command voltage fed from the command power supply 7, and thereby controls base current of the transistor 2. 9 is a load connected between the output terminals 3 and 4.
The operations of the foregoing DC-DC converter of prior art are explained below.
Output voltage between output terminals 3 and 4 is divided and detected by resistors 5 and 6. The detected voltage is inputted to a differential amplifier 8. The differential amplifier 8 compares the detected voltage with the command voltage fed from a command power supply 7, then amplifies the difference between the voltages to control base current of a transistor 2. The collector-to-emitter voltage of the transistor 2 varies depending on the base current. Thus, the transistor 2 can control output voltage. In this case, it is impossible to make an output voltage higher than an input voltage fed from a dc power supply 1. However, input voltage can be varied in response to considerably high-frequencies. (Thus, the DC-DC converter can act on rapid variations in command voltage).
FIG. 16 shows another DC-DC converter of the prior art. The circuit diagram shows a switching power supply based on a forward converter. In FIG. 16, 1 denotes a dc power supply, and 9, a load. 10 and 11 are high-potential and low-potential output terminals, respectively, to which the load 9 is connected.
12 and 13 are resistors for detecting output voltage connected in series between the output terminals 10 and 11. 14 denotes a command power supply containing command voltage. 15 is a differential amplifier for comparing the divided value of output voltage developed at the junction of the resistors 12 and 13 with the command voltage fed from the command power supply 14. 16 is a photocoupler for electrically insulating the output of the differential amplifier 15.
17 denotes a pulse width control circuit connected to the output of the photocoupler 16, and 18, a switching element whose on and off operations are controlled according to the output of the pulse width control circuit 17. 19 is a transformer having a primary winding 19a, a secondary winding 19b, and a reset winding 19c connected in series with the primary winding 19a. The junction of the primary winding 19a and reset winding 19c is connected to the positive terminal of the dc power supply 1. The other side of the primary winding 19a is connected to the output of the switching element 18, and one side of the secondary winding 19b, to the low-potential output terminal 11. 20 denotes a diode whose anode is connected to the negative terminal of the dc power supply 1 and cathode is connected to the reset winding 19c of the transformer 19.
21 and 22 are diodes, wherein the cathodes are connected to a common node and the anodes are connected to the sides of the secondary winding 19b of the transformer 19. 23 denotes a choke coil connected between the cathodes of the diodes 21 and 22, and the high-potential output terminal 10. 24 denotes a smoothing capacitor connected between the high-potential output terminal 10 and the low-potential output terminal 11.
The operations of the foregoing DC-DC converter of prior art are explained below.
Intermittent energy fed from a dc power supply 1 by the switching operation of a switching element 18 is rectified by diodes 21 and 22 connected to a secondary winding 19b of a transformer 19. Then, the energy is smoothed by a choke coil 23 and a smoothing capacitor 24, then provided as a DC output between output terminals 10 and 11.
At this time, energy transfer to the secondary winding 19b of the transformer 19 is done with the switching element 18 on. A reset winding 19c of the transformer 19 is designed to restore energy stored in the transformer 19 with the switching element 18 on to the dc power supply 1 via a reset diode 20 with the switching element 18 off.
On the other hand, output voltage is controlled as follows; that is, the difference between the voltage divided and detected by resistors 12 and 13 and the command voltage fed from a command power supply 14 is amplified by a differential amplifier 15, then provided to a photocoupler 16. A pulse width control circuit 17 receives the signal via the photocoupler 16, then controls the on and off operations of the switching element 18 at the on/off ratio corresponding to the signal received. Thereby, the input voltage from the dc power supply 1 is stepped up or down according to the command voltage, then provided as output voltage between output terminals 10 and 11.
DC-DC converters of prior art have the aforesaid configuration. Therefore, a series regulator cannot make (step up) an output voltage higher than an input voltage. In addition, it is difficult to extend the range of frequencies at which a switching power supply can vary output voltage because of the restriction of the switching frequency or the cutoff frequency of a filter for smoothing outputs (the switching power supply cannot act on rapid variations in command voltage). Even when a DC-DC converter has a configuration shown in FIG. 17 that a booster-type switching power supply 25 and a series regulator 26 are connected in tandem (in series), the overall power conversion efficiency greatly deteriorates because it is a product of the efficiencies of the booster-type switching power supply 25 and the series regulator 26.